Medical device position guidance system with wireless connectivity between a noninvasive and an invasive device

CROSS REFERENCES TO RELATED APPLICATIONS

0001This application is continuation that claims priority to U.S. patent application Ser. No. 11/530,385 filed on Sep. 8, 2006, the entire contents of which being incorporated herein by reference.

BACKGROUND

0002Physicians and other health care providers frequently use catheters to treat patients. The known catheters include a tube which is inserted into the human body. Certain catheters are inserted into through the patient's nose or mouth for treating the gastrointestinal tract. These catheters, sometimes referred to as enteral catheters, typically include feeding tubes. The feeding tube lies in the stomach or intestines, and a feeding bag delivers liquid nutrient, liquid medicine or a combination of the two to the patient.

0003Other types of catheters are inserted into the patient's veins or arteries for treating the cardiovascular system. These intravascular catheters include, among others, the central venous catheter, peripheral venous catheter and the peripherally inserted central catheter (PICC). These catheters include a relatively small tube that passes through the patient's veins or arteries. Depending on the application, the health care providers can use these intravascular catheters to remove blood vessel blockages, place inserts into blood vessels and to provide patients with injections of medications, drugs, fluids, nutrients, or blood products over a period of time, sometimes several weeks or more.

0004When using these known enteral and intravascular catheters, it is important to place the end of the catheter at the proper location within the human body. Erroneous placement of the catheter tip may injure or harm the patient. For example, if the health care provider erroneously places an enteral catheter into the patient's lungs, liquid may be introduced into the lungs with harmful results. If the health care provider erroneously places an intravascular catheter into the wrong blood vessel of the cardiovascular system, the patient may experience infection, injury or a harmful blockage.

0005In some cases, health care providers use X-ray machines to gather information about the location of the catheters within the body. There are several of disadvantages with using X-ray machines. For example, these machines are relatively large and heavy, consume a relatively large amount of energy and expose the patient to a relatively high degree of X-ray radiation. Also, these machines are typically not readily accessible for use because, due to their size, they are usually installed in a special X-ray room. This room can be relatively far away from the patient's room. Therefore, health care providers can find it inconvenient to use these machines for their catheter procedures. Furthermore, it can be inconvenient to transport these machines to a patient's home for home care catheter procedures.

0006Accordingly, there is a need to overcome or otherwise lessen the effects of such disadvantages.

SUMMARY

0007A medical device position guidance system having a noninvasive medical device communicable directly or indirectly with an invasive medical device. The system provides visual or audio output useful to assess the position of the invasive medical device in an animal, such as a human, with respect to the position of the noninvasive medical device. A magnetic field is used to gather information about the position of the invasive device relative to the noninvasive device. Radio waves are used to communicate this information between the noninvasive device and the invasive device.

BRIEF DESCRIPTION OF THE FIGURES

<figref idref="DRAWINGS">FIG. 1</figref> is a top or plan view of the medical device position guidance system illustrating an intravenous application involving a peripherally inserted central catheter inserted into a human body and illustrating radio wave communication between the catheter assembly and the noninvasive device.

<figref idref="DRAWINGS">FIG. 2</figref> is a perspective view of the medical device position guidance system illustrating an enteral application involving a catheter inserted into a human body and illustrating communication between the catheter assembly and the noninvasive device.

<figref idref="DRAWINGS">FIG. 3</figref> is an elevated side and schematic view of an embodiment of a catheter assembly communicating with the noninvasive device, where the noninvasive device uses a vertically-oriented output coil.

<figref idref="DRAWINGS">FIG. 4</figref> is a perspective view of the noninvasive device, according to an embodiment.

<figref idref="DRAWINGS">FIG. 5</figref> is a top view of the noninvasive device, according to an embodiment.

<figref idref="DRAWINGS">FIG. 6</figref> is a rear view of the noninvasive device, according to an embodiment.

<figref idref="DRAWINGS">FIG. 7</figref> is a top and schematic view of the enteral catheter assembly, according to an embodiment.

<figref idref="DRAWINGS">FIG. 8</figref> is an enlarged top view of the enteral catheter illustrating the coil in the bolus of the catheter assembly of <figref idref="DRAWINGS">FIG. 7</figref>.

<figref idref="DRAWINGS">FIG. 9</figref> is a schematic view of a catheter assembly having a catheter with an open end.

<figref idref="DRAWINGS">FIG. 10</figref> is a schematic view of a catheter assembly having a catheter with flapped openings on the sidewalls of the catheter.

<figref idref="DRAWINGS">FIG. 11</figref> is a schematic view of a catheter assembly having a medical treatment device disposed at an end of the catheter for use in ablation therapy or other types of medical treatment.

<figref idref="DRAWINGS">FIG. 12</figref> is a schematic view of a catheter assembly having a stent and a balloon at an end of the catheter for use in stenting operations.

<figref idref="DRAWINGS">FIG. 13</figref> is an enlarged top, schematic view of the noninvasive portion of the catheter assembly.

<figref idref="DRAWINGS">FIG. 14A</figref> is a schematic view of the medical device position guidance system illustrating an embodiment where the invasive device and the noninvasive device communicate indirectly with one another through a main processing unit.

<figref idref="DRAWINGS">FIG. 14B</figref> is a schematic view of the medical device position guidance system illustrating an embodiment where the invasive device and the noninvasive device communicate indirectly with one another through a main processing unit.

<figref idref="DRAWINGS">FIG. 15</figref> is a simplified schematic block diagram of one embodiment of a catheter locating system.

<figref idref="DRAWINGS">FIG. 16A</figref> is a schematic diagram of the pair of horizontal field generating coils located over the catheter of the system of <figref idref="DRAWINGS">FIG. 15</figref>, useful in illustrating the catheter depth determination accomplished by the system of this embodiment.

<figref idref="DRAWINGS">FIG. 16B</figref> is a graph of sensor coil voltage induced by the pair of horizontal field generating coils of <figref idref="DRAWINGS">FIGS. 15 and 16A</figref> at four different catheter depths as the horizontal field generating coils are moved horizontally along the patient's skin.

<figref idref="DRAWINGS">FIG. 17</figref> is a view similar to that of <figref idref="DRAWINGS">FIG. 16A</figref>, illustrating the relative horizontal angular orientation of the sensor coil longitudinal axis, the longitudinal axes of the two horizontal field generating coils and the phases of the induced sensor voltages relative to the phases of the respective output coil drive voltages;

<figref idref="DRAWINGS">FIG. 18A</figref> is a cross sectional, schematic view of a catheter and one embodiment of the external noninvasive device of this invention that uses a vertically-oriented output coil.

<figref idref="DRAWINGS">FIG. 18B</figref> is an alternate form of an output coil set incorporating a vertically-oriented output coil.

<figref idref="DRAWINGS">FIG. 19A</figref> is a view of the sensor showing an edge view of the plane which bisects the sensor midpoint.

<figref idref="DRAWINGS">FIG. 19B</figref> is a graph of sensor coil output voltage induced by the vertical coil of <figref idref="DRAWINGS">FIG. 18A</figref> versus the distance of the vertical coil from the plane bisecting the sensor coil and perpendicular to the sensor coil longitudinal axis for three different sensor depths.

<figref idref="DRAWINGS">FIG. 20</figref> is a schematic electronic diagram of the embodiment of the system of this invention.

<figref idref="DRAWINGS">FIG. 21</figref> is a timing diagram for the circuit of <figref idref="DRAWINGS">FIG. 20</figref>.

<figref idref="DRAWINGS">FIG. 22</figref> depicts the multiplexer output for the system of <figref idref="DRAWINGS">FIG. 20</figref>, showing reference, battery, test sensor and sensor coil voltages.

<figref idref="DRAWINGS">FIG. 23</figref> is a flow chart illustrating the programming of the microprocessor of <figref idref="DRAWINGS">FIG. 20</figref>, and the operation of the system of <figref idref="DRAWINGS">FIG. 20</figref>.

<figref idref="DRAWINGS">FIG. 24A</figref> is a top plan view of one form of an noninvasive device for the system of this invention.

<figref idref="DRAWINGS">FIG. 24B</figref> is an enlarged view of the distance display of the noninvasive device of <figref idref="DRAWINGS">FIG. 24A</figref>.

<figref idref="DRAWINGS">FIG. 25</figref> is a top plan view of an alternative design for the noninvasive device of the system of this invention.

DETAILED DESCRIPTION

I. General Overview

0038Referring now to the drawings, <figref idref="DRAWINGS">FIG. 1</figref> illustrates one embodiment of a medical device position guidance system <b>500</b>. The medical device position guidance system <b>500</b> includes an external device or noninvasive device <b>502</b> and a catheter assembly <b>504</b>. The noninvasive device <b>502</b> is positionable over a surface of an animal, such as a human <b>506</b>. Although the illustrated example depicts a human, it should be appreciated that medical device position guidance system <b>500</b> could be used with any animal such as domestic animals. In general, the noninvasive device <b>502</b> includes a noninvasive housing <b>508</b> which supports a magnetic field generator <b>510</b>, and a receiver <b>512</b> including an antenna <b>512</b><i>a </i>operably coupled to a processor <b>523</b>, where the processor <b>523</b> is coupled to a memory device <b>515</b>. According to the embodiments, the medical device position guidance system <b>500</b> is operable to provide audiovisual information about the orientation of an invasive medical device and the position of the invasive medical device relative to the external device <b>502</b>, through a wireless connection between the invasive medical device <b>504</b> and the noninvasive device <b>502</b>.

II. Intravascular Embodiment

0039In an embodiment of the medical device position guidance system <b>500</b>, the magnetic field generator <b>510</b> includes a plurality of electromagnetic output coils <b>558</b>, as illustrated in <figref idref="DRAWINGS">FIG. 3</figref>. As illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, the user inserts a predetermined length of the catheter assembly <b>504</b> into the patient's blood vessel <b>532</b>. In one example, an outer wall of the catheter includes markings indicating the length of the catheter <b>522</b> that has been inserted.

0040The user or medical personnel moves the noninvasive device <b>502</b> over the skin <b>518</b> of the human body <b>506</b> and powers-on the magnetic field generator <b>510</b> to generate a magnetic field. The user moves the noninvasive device <b>502</b> to direct the generated magnetic field through the skin <b>518</b> of the human body <b>506</b> to a location that is proximate to the estimated position of the end or tip <b>520</b> of the catheter <b>522</b>. The receiver <b>512</b> receives radio wave signals from the transmitter <b>516</b> of the catheter assembly <b>504</b>, as described in further detail below. In an embodiment, the receiver <b>512</b> includes an antenna <b>512</b><i>a</i>, as illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, to receive radio frequency signals and is connected to a processor <b>523</b> that is powered by a battery <b>521</b>.

0041In an embodiment, the catheter assembly <b>504</b> includes a catheter <b>522</b> that is sized and shaped to be inserted into an animal. A distal end <b>528</b> of the catheter <b>522</b> may be inserted intravenously as shown in <figref idref="DRAWINGS">FIG. 1</figref> into a vein, artery or blood vessel <b>532</b> of the human body <b>506</b> or enterally as shown in <figref idref="DRAWINGS">FIG. 2</figref> into the gastrointestinal tract. In one example illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, the user or medical personnel inserts the distal end <b>528</b> of the catheter <b>522</b> through the arm vein <b>532</b> to a position adjacent to the heart <b>534</b> of the body <b>506</b> of a patient. In an embodiment, the catheter assembly <b>504</b> includes a y-port connector <b>536</b>. Branched end <b>542</b> of the y-port connector <b>536</b> connects, directly or indirectly, to a fluid source <b>540</b>. The fluid source <b>540</b> may be medicine or any other suitable fluid used in intravenous or intravascular medical procedures. It should be appreciated that the branched end <b>542</b> of the y-port connector <b>536</b> may be closed off by an end cap <b>546</b> depending upon the medical procedure that is implemented. The branched end <b>544</b> of the y-port connector <b>536</b> is matably connectable through connector <b>538</b> to the extension <b>530</b> of the catheter <b>522</b>. Referring to <figref idref="DRAWINGS">FIG. 1</figref>, the extension <b>530</b> of the catheter <b>522</b> includes a noninvasive portion carrying an antenna <b>560</b>, as shown in <figref idref="DRAWINGS">FIG. 13</figref>, a processor <b>548</b>, one or more indicators <b>550</b> and a battery <b>552</b>. The battery <b>552</b> supplies power to the indicator <b>550</b>, the processor <b>548</b> and the antenna <b>560</b>. The catheter assembly <b>504</b> also includes an elongated conductor <b>554</b> connected to the processor <b>548</b> which runs through the extension <b>530</b> of the catheter <b>522</b>, through the connector <b>538</b>, through the y-port assembly <b>536</b>, and continues to a coil <b>524</b> at the distal end <b>528</b> of the catheter.

0042In one embodiment, a stylet is operatively connected to the transmitter <b>516</b> and includes a tube and a guidewire supported inside the tube. The guidewire includes a steering wire or core wire and an elongated conductor wire. In an embodiment, the steering wire or core wire has a stiffness characteristic to facilitate with steering of the tip of the tube through a passageway in an animal. In an embodiment, the conductor wire is a single wire having a positive end and a negative end, which are positioned adjacent to each other at the proximate end of the core wire. Along the length of the core wire, the ends of the conductor wire are twisted about the core wire to shield or reduce any magnetic fields generated by the conductor wire along the length of the core wire. At the distal end of the core wire, the conductor wire forms a helical coil configured to induce a current when exposed to a magnetic field. In an embodiment, the core wire also functions as a grounding device for the antenna <b>560</b> of the transmitter <b>516</b>, as shown in <figref idref="DRAWINGS">FIG. 13</figref>. Therefore, the antenna's ground device includes the core wire. In other embodiments, a suitable grounding device is incorporated directly into the antenna.

0043In another embodiment, the conductor <b>554</b> includes at least one conductor wire <b>554</b><i>a </i>axially twisted about a relatively stiff guidewire, and these wires function as a stylet, aiding the user in steering the catheter <b>522</b> inside the human <b>506</b>. The coil <b>524</b> is, in one embodiment, comprised of a helical structure formed by multiple spirals of the distal end <b>528</b> of the conductor wire <b>554</b><i>a</i>. However, it should be appreciated that, in other embodiments, the coil <b>524</b> can be a separate wire coil unit which is operatively coupled to the conductor <b>554</b> in any suitable fashion. It should also be appreciated that, in another embodiment, the antenna <b>560</b>, processor <b>548</b> and battery <b>552</b> can be housed, lodged or otherwise incorporated into the walls of the distal end <b>528</b> of the catheter <b>522</b>.

0044Referring to <figref idref="DRAWINGS">FIG. 1</figref>, in one embodiment, when the connector <b>538</b> is disconnected from the y-port assembly <b>536</b>, the conductor <b>554</b> and the coil <b>524</b> can be withdrawn through the distal end <b>528</b> of the catheter <b>522</b> out of the vessel <b>532</b> of the patient while the catheter <b>522</b> remains inside the patient. In an example, the user removes the conductor <b>554</b> and sensor coil <b>524</b> from the body after the medical device position guidance system <b>500</b> assesses the actual or approximate position and orientation of the tip or end <b>520</b> of the catheter <b>522</b> in the patient. In this example, after the wire <b>554</b> and sensor coil <b>524</b> are withdrawn, fluid can be introduced from the fluid source <b>540</b> or other medical treatment can be performed at the treatment site inside the vessel <b>532</b>.

0045In operation, in an embodiment, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, the user or medical personnel: (a) inserts a catheter a known or approximate distance into the vessel <b>532</b> of the patient; (b) activates the noninvasive device <b>502</b> to begin generating the magnetic field; (c) moves the noninvasive device <b>502</b> to direct the generated magnetic field <b>568</b> as shown in <figref idref="DRAWINGS">FIG. 3</figref> in the proximity of where the tip <b>520</b> of the catheter <b>522</b> is estimated to be located; (d) receives an indication from the noninvasive device <b>502</b> regarding the relative position and orientation of the catheter tip <b>520</b> with respect to the noninvasive device <b>502</b>; and (e) adjusts the position of the noninvasive device <b>502</b> or the catheter assembly <b>504</b> or both until the user is comfortable with the location of the catheter tip <b>520</b> within the vessel <b>532</b>.

0046In an embodiment, the noninvasive device <b>502</b> includes a main battery <b>556</b> that provides a voltage to the plurality of electromagnetic output coils <b>558</b>, as illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, to produce the magnetic field. Referring to <figref idref="DRAWINGS">FIGS. 1 and 3</figref>, the magnetic field propagates through the tissue or skin <b>518</b> of the patient <b>506</b> and induces a voltage in the coil <b>524</b>. The induced voltage travels through the wire <b>554</b> to the processor <b>548</b> of the transmitter assembly <b>558</b>. The processor <b>548</b> of the transmitter assembly <b>558</b> converts the induced voltage and supplies information to the transmitter <b>516</b>. The transmitter <b>516</b> wirelessly outputs the information through an antenna <b>560</b> as illustrated <figref idref="DRAWINGS">FIG. 13</figref> in the form of modulated electromagnetic waves or radio waves. In one example, the frequency of the radio waves is approximately seventy hertz. However, it should be appreciated that the frequency may be any suitable frequency to allow radio communication between the transmitter and the receiver.

0047As illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, the radio waves wirelessly travel through the communication channel <b>564</b> or atmosphere to an antenna <b>512</b><i>a </i>of the receiver <b>512</b> of the noninvasive device <b>502</b>. In an embodiment, the transmitter assembly <b>558</b> also includes one or more indicators <b>550</b> that indicate, upon instruction from the processor <b>552</b>, whether the information has been successfully transmitted. However, the indicators <b>550</b> may also be used to indicate other operating parameters such as battery <b>552</b> strength, for example. The induced voltage in the coil <b>524</b> provides information regarding the proximity of the tip <b>520</b> of the catheter <b>522</b> to the noninvasive device <b>502</b> over the surface of the human. The induced voltage also provides information regarding the directional orientation of the tip <b>520</b> of the catheter <b>522</b> relative to the noninvasive device <b>504</b>. The receiving processor <b>523</b>, as illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, receives information, transmitted by the transmitter <b>516</b> of the transmitter assembly <b>558</b>, and the receiver processor <b>523</b> converts this information from a sinusoidal electromagnetic wave having a determined modulated frequency to a series of electrical impulses. The receiver processor <b>523</b> sends these impulses to the main processor <b>517</b> through one or more pins within the receiver <b>512</b>. The main processor <b>517</b>, as directed by the instructions of memory device <b>515</b>, processes these impulses to cause the indicator <b>566</b> of the noninvasive device as illustrated in <figref idref="DRAWINGS">FIG. 5</figref> to provide visual output in the form of the selection illumination of the directional arrow light sources <b>578</b>. The arrow lights <b>578</b> indicate whether the tip <b>520</b> is pointed North, East, South, West or in between such directions. The main processor <b>517</b> also causes a speaker <b>586</b> to generate a series of variable tones. The main processor <b>517</b> varies the series of tones by frequency, volume or pitch to indicate whether the user is moving the noninvasive device <b>502</b> closer to, or further away from, the tip <b>520</b> in the human body. It should be appreciated that, the indicators <b>566</b> can include one or more light emitting diodes (LEDs) or display devices, such as liquid crystal display (LCD) panels operable to provide graphics and images related to the position of the tip <b>520</b>.

0048The noninvasive device <b>502</b> is operatively connectable to, and in communication with, the catheter assembly <b>504</b> wirelessly through the two antennas <b>516</b> and <b>512</b>. Therefore, in this embodiment, the noninvasive device <b>502</b>, has no lead wires or cables physically connecting the noninvasive device <b>502</b> to the catheter assembly <b>504</b>. As such, the noninvasive device <b>502</b> can be fully contained within a disposable sterile bag or envelope to protect the patient from contamination that may arise with respect to cross-patient uses of the noninvasive device <b>502</b>. Accordingly, the noninvasive device <b>502</b> may be reused over a series of procedures with different patients while facilitating sterility. Also, the lack of wires attached to the exterior of the noninvasive device <b>502</b> minimizes or reduces the possibility of wires becoming tangled with the medical personnel or other equipment during a medical procedure.

0049In an embodiment, with reference to <figref idref="DRAWINGS">FIGS. 4 to 6</figref>, the receiver <b>512</b> of the noninvasive device <b>502</b> is attachable to the main housing <b>580</b> of the noninvasive device <b>502</b> as a removable modular unit <b>582</b>. In an embodiment, this modular unit <b>582</b> includes a dedicated battery <b>521</b> as shown in <figref idref="DRAWINGS">FIG. 3</figref>, and one or more electrical connections to removably connect to corresponding electrical connections located on the main housing <b>580</b>. For example, the electrical connections may be a multiple pin and socket arrangement. In this embodiment, the processor <b>523</b> converts the modulated radio waves received through the antenna <b>560</b>, as shown in <figref idref="DRAWINGS">FIG. 13</figref> into electrical impulses, and the electrical impulses are transmitted through the electrical connectors. The electrical impulses are interpreted by a the main processor <b>517</b> housed in the main housing <b>580</b>, as illustrated in <figref idref="DRAWINGS">FIGS. 3 and 4</figref>. The main processor <b>517</b> controls the indicators <b>566</b> based on this information. The noninvasive device <b>502</b> includes a power switch <b>584</b>, illustrated in <figref idref="DRAWINGS">FIG. 4</figref>, that activates the battery in the main housing <b>580</b> to provide power to the main processor <b>517</b> and indicators <b>566</b> of the noninvasive device. In the embodiment where the modular unit <b>582</b> has a secondary battery or power source, the power switch <b>584</b> may also activate the secondary battery. However, it should be appreciated that the modular unit <b>582</b> may also include a separate power switch to activate the secondary battery.

0050In one embodiment, the modular unit <b>582</b> has a plurality of internal walls configured to mate with walls of the main housing <b>580</b> in a press-fit connection. By removing or attaching the unit <b>582</b>, the user can convert the noninvasive device <b>502</b> between: (a) wireless communication mode in which the device <b>502</b> communicates with the catheter assembly <b>504</b> through antenna <b>516</b> and <b>512</b> and (b) a mode in which the device <b>502</b> is physically connected to the catheter assembly <b>504</b> through a data cable.

0051Referring to <figref idref="DRAWINGS">FIG. 13</figref>, in an embodiment, the transmitter assembly <b>558</b> includes a battery <b>552</b>, a processor <b>548</b>, an indicator <b>550</b>, and a transmitter <b>516</b>. The transmitter assembly <b>558</b> also includes a rotatable plug or cap <b>632</b> that is operable to electrically connect the battery <b>552</b> to the processor <b>548</b>. In one embodiment, the rotatable cap <b>632</b> includes a threaded connector that can be screwed onto a portion of the noninvasive housing <b>508</b> and has an electrical contact that is movable between: (i) a first position in which the battery <b>552</b> is electrically disconnected from the processor <b>548</b>; and (ii) a second position in which the battery <b>552</b> is electrically connected to the processor <b>548</b>. In one example, the battery <b>552</b> is initially in a non-activated state before the catheter assembly <b>522</b> has been used and the user screws on the rotatable cap <b>632</b> to an end portion <b>636</b> of the transmitter assembly <b>558</b> to engage the contact <b>634</b> with an electrical contact of the processor <b>548</b> to allow the battery to provide power to the processor <b>548</b>, the indicator <b>550</b> and the transmitter <b>516</b>. As mentioned above, the transmitter <b>516</b> converts the electrical impulses sent through the wire <b>554</b> into modulated radio waves and emits the radio waves out through the antenna <b>560</b>. In other embodiments, the electrical contact <b>634</b> may be a removable insulating film or other suitable device operable to cause the battery circuit to close when the film is removed from the circuit.

0052With certain invasive medical procedures, it is desirable to dispose of and incinerate the catheter assembly <b>504</b> because portions of the catheter assembly <b>504</b> have contacted bodily fluids such as blood or gastrointestinal fluids. It is often desirable to dispose of the battery <b>552</b> rather than attempting to incinerate the battery <b>552</b>. In one embodiment, the battery <b>552</b> is housed in the rotatable cap <b>632</b> which can rotate in the opposite direction or be unscrewed to completely remove the battery <b>552</b> and the rotatable cap <b>632</b> from the catheter assembly <b>504</b>. In this embodiment, the rotatable cap <b>632</b> facilitates disposal of the battery separate from the catheter assembly.

0053Referring to <figref idref="DRAWINGS">FIG. 14A</figref>, in an embodiment, the transmitter <b>516</b> of catheter assembly <b>504</b> is configured to communicate indirectly with the receiver <b>512</b> of the noninvasive device <b>502</b>. In one example, the transmitter <b>516</b> transmits radio waves through antenna <b>560</b> to a receiver <b>638</b> of a central processing unit <b>640</b>. The receiver <b>638</b> of the central processing unit <b>640</b> converts the modulated radio waves into electrical impulses that are supplied to the processor of the central processing unit <b>640</b>. In an example, the central processing unit <b>640</b> is a computer having a processor and a display device <b>644</b>. In this embodiment, the display device <b>644</b> displays information regarding the position and orientation of the medical device in the body of the patient relative to the noninvasive device <b>502</b>. The central processing unit <b>640</b> also includes a transmitter <b>642</b> to communicate with the receiver <b>512</b> of the noninvasive device <b>502</b>.

0054Referring to <figref idref="DRAWINGS">FIG. 14B</figref>, in an embodiment, medical device position guidance system <b>701</b> includes a noninvasive device <b>702</b> and a medical device <b>704</b>. In this embodiment, the medical device <b>704</b> is configured to communicate indirectly with the noninvasive device <b>702</b> through a central processing unit <b>744</b>. As shown in <figref idref="DRAWINGS">FIG. 14B</figref>, medical device <b>704</b> includes: (a) a catheter <b>722</b> having an invasive end or tip <b>720</b>; (b) a receiver housing <b>758</b> that houses a receiver <b>716</b> having an antenna <b>760</b>, a battery <b>748</b>, a processor <b>750</b>, and an indicator <b>752</b>; (c) a conductor <b>754</b> supported by the catheter <b>722</b> and operatively connected to the receiver <b>716</b>; and (d) an invasive magnetic field generator <b>724</b> operatively connected to an invasive distal end <b>770</b> of the conductor <b>724</b> and powered by the battery <b>748</b>.

0055The noninvasive device <b>702</b> includes: (a) a transmitter or transceiver <b>712</b> having an antenna <b>772</b>; (b) a battery <b>780</b>; (c) a processor <b>782</b>; (d) an indicator <b>784</b>; and (e) one or more coils <b>710</b> operatively connected to the transmitter <b>712</b> through the processor <b>774</b>. The coils are operable to receive an induced current in response to a magnetic field <b>768</b> generated by the magnetic field generator <b>724</b> when the magnetic field <b>768</b> is directed toward and reaches the coils <b>710</b>. It should be appreciated that the coils <b>710</b> may be any suitable structure or structures capable of receiving a current in response to a generated magnetic field. The central processing unit <b>740</b> includes a display <b>744</b>, a transceiver or receiver <b>742</b> having an antenna <b>788</b> and a transmitter or transceiver <b>738</b> having an antenna <b>790</b>. It should be appreciated that although the transmitter <b>738</b> and receiver <b>742</b> are illustrated to be separate components, they may be the same component functioning as a transceiver and sharing an antenna.

0056In one embodiment, referring to <figref idref="DRAWINGS">FIG. 14B</figref>, in operation: (a) the central processing unit <b>740</b> causes the transmitter <b>738</b> to emit radio waves through the antenna <b>790</b>; (b) the receiver <b>716</b> of the medical device <b>704</b> receives the radio waves through the antenna <b>760</b> and the processor <b>750</b> converts the radio waves to a series of electrical impulses; (c) the electrical impulses travel through the conductor <b>754</b> to the magnetic field generator <b>724</b>; (d) the magnetic field generator <b>724</b> generates a magnetic field <b>768</b> that passes through the tissue <b>718</b> of the animal and induces a current in the coils <b>710</b> of the noninvasive device; (e) the processor <b>782</b> converts the induced current to radio waves that are emitted through the antenna <b>772</b> of the transmitter <b>712</b>; and (f) the receiver <b>742</b> of the central processing unit <b>744</b> receives the radio waves through the antenna <b>788</b>. In this embodiment, the display device <b>744</b> displays information regarding the position, path or shape of path, or orientation of the invasive portion of the medical device <b>704</b> in the body of the patient.

III. Enteral Application

0057In an embodiment, as illustrated in <figref idref="DRAWINGS">FIG. 2</figref>, the medical device position guidance system <b>501</b> is utilized in an enteral application. Here, the system <b>501</b> includes the same components, elements, structure and functionality as system <b>500</b> except that system <b>501</b> includes enteral catheter <b>523</b> and enteral fluid source <b>541</b> instead intravascular catheter <b>522</b> and intravascular catheter <b>522</b> and intravascular fluid source <b>540</b>, respectively. As illustrated in <figref idref="DRAWINGS">FIGS. 7 and 8</figref>, the enteral catheter <b>523</b> has a partially rounded end, tip <b>520</b> or bolus. The bolus has a plurality of lowered side walls which define an upper opening. In one embodiment, the bolus defines an additional opening at its end.

0058In this application, the user or medical personnel positions the noninvasive device <b>502</b> over the skin or tissue <b>518</b> of a patient. In this enteral application of the medical device position guidance system <b>501</b>, the distal end <b>528</b> of the catheter <b>523</b> is inserted through the mouth and esophagus <b>572</b> into the enteral cavity <b>570</b> of a patient. The branched arm <b>542</b> of the y-port connector <b>536</b> connects to a fluid source <b>541</b> such as a feeding bag. The medical device position guidance system <b>501</b> provides the user with audiovisual information to assist in assessing the position of the end or tip <b>520</b> of the catheter <b>523</b> as described above with respect to the intravascular system <b>500</b>. As such, the enteral system <b>501</b> reduces the risk that the catheter may be inadvertently misplaced. Accordingly, the risk of injury to the patient is reduced.

0059Referring to <figref idref="DRAWINGS">FIGS. 7-8</figref>, in one embodiment, the multi-port or y-port connector <b>536</b> includes: (a) a body <b>594</b>; (b) a first branched arm or liquid delivery branch, medicine delivery branch or medicine branch <b>542</b> attached to the body <b>594</b> for distributing drugs, medicine or other medicinal liquids to the patient; (c) a second branched arm or catheter connection branch <b>544</b> attached to the catheter <b>522</b>; (d) a flexible or movable arm <b>592</b> attached to the body <b>594</b>; and (f) a flexible or moveable arm <b>546</b> attached to the body <b>594</b>. In an alternative embodiment, y-port connector <b>536</b> includes additional branches for administering various nutrients or medicines to the body. In another alternative embodiment, the y-port connector <b>536</b> includes only a feeding branch <b>542</b> and a connection branch <b>544</b>. The arm <b>546</b> has a stopper <b>596</b>, and the arm <b>544</b> has a stopper <b>598</b>. The stoppers <b>596</b> and <b>598</b> are sized to prevent fluid from passing through the branches <b>544</b> and <b>542</b> after such branches <b>544</b> and <b>542</b> are plugged with stoppers <b>596</b> and <b>596</b>, respectively. In addition, the arm <b>544</b> includes a tube-size adapter <b>600</b> to the arm <b>544</b>. The tube-size adapter <b>600</b> enables fluid delivery tubes (not shown) having various diameters to connect to the feeding branch <b>544</b> of the y-port connector <b>536</b>.

0060As illustrated in <figref idref="DRAWINGS">FIGS. 7-8</figref>, in one embodiment, the enteral catheter <b>523</b> includes a feeding tube having: (a) a proximal end <b>602</b> attached to the catheter connection branch <b>544</b> of the y-port connector <b>536</b>; (b) a distal end; and (c) an external surface <b>604</b>. The proximal end <b>602</b> is insertable into the catheter connection branch <b>544</b> of the y-port connector <b>536</b> so as to bring the enteral catheter <b>523</b> into fluidic communication with the y-port connector <b>536</b>. In one embodiment, the external surface <b>604</b> has a plurality of volumetric, measurement or unit markings (not shown) uniformly spaced along enteral catheter <b>523</b>. These markings assist the user in measuring the flow or distribution of liquid to or from the patient. In an alternative embodiment, markings function as placement markers which assist the user in assessing the depth that the catheter is placed within the human body. In this embodiment, the markings may be used to make the initial estimation as to where the tip <b>520</b> or end of the catheter <b>523</b> is within the patient or body. Then the medical device positioning guidance system <b>500</b> is used to provide audiovisual information about the position of the tip <b>520</b>.

0061As best illustrated in <figref idref="DRAWINGS">FIG. 7</figref>, in one embodiment, the conductor <b>554</b> connects to the coil <b>524</b> and runs through the catheter <b>522</b>, through the first branched arm <b>544</b> of the y-port connector <b>536</b> and on to the transmitter assembly <b>558</b>. As described above, the electrical impulses induced in the coil <b>524</b> are sent along the wire and converted by the transmitter <b>516</b> into modulated radio waves.

0062As best illustrated in <figref idref="DRAWINGS">FIG. 8</figref>, in one embodiment, the end member, bolus or tip <b>520</b> is attached to the distal end <b>606</b> of the catheter <b>522</b>. As described above, the tip <b>520</b> includes a body <b>608</b> having a collar <b>610</b> and an end member <b>612</b>. The body <b>608</b> defines a passage <b>616</b> and an opening <b>618</b>. The opening <b>618</b> is positioned between the collar <b>610</b> and the end member <b>612</b>. A portion <b>614</b> of the end member <b>612</b> can have a rounded shape. The shape of the passage <b>616</b> and opening <b>618</b> of the tip <b>520</b> is configured to facilitate the flow of fluid from the catheter <b>522</b> into the patient's body while decreasing the likelihood that the opening <b>618</b> will become clogged.

IV. Other Applications

0063It should be appreciated that the noninvasive device <b>502</b> and transmitter assembly <b>558</b> can be used together in a variety of medical applications. In one embodiment, as illustrated in <figref idref="DRAWINGS">FIG. 9</figref>, the transmitter assembly <b>558</b> is coupled to a catheter with an opening <b>618</b> located on the portion <b>614</b> of the end member <b>612</b>. In this embodiment, the end member <b>612</b><i>a </i>is tubular and may be cut such that the opening <b>618</b> is circular and defined by the tube diameter. In another embodiment, as illustrated in <figref idref="DRAWINGS">FIG. 10</figref>, the transmitter assembly <b>558</b> is coupled to a catheter that defines one or more openings <b>618</b> located on a sidewall <b>620</b> of the end member <b>612</b><i>b</i>. In this embodiment, the sidewall <b>620</b> is cut to form flaps <b>622</b> that allow fluid to be dispensed to the patient.

0064In another embodiment, as illustrated in <figref idref="DRAWINGS">FIG. 11</figref>, the transmitter assembly <b>558</b> is coupled to an end member <b>612</b><i>c </i>through a wire <b>554</b>. In this embodiment, the end member includes a medical device <b>624</b> such as a radio frequency or thermal energy ablation device. In one example, the medical device <b>624</b> does not include a catheter, and thus, it not configured to deliver fluid to the patient. In this embodiment, the coil <b>524</b> transmits electrical impulses through the wire <b>554</b> to the transmitter assembly <b>558</b>, as discussed above.

0065In one embodiment, as illustrated in <figref idref="DRAWINGS">FIG. 12</figref>, the transmitter assembly <b>558</b> is coupled to the end member <b>612</b><i>d </i>through a wire <b>554</b>, as discussed above. In this embodiment, the end member <b>612</b><i>d </i>includes a stent <b>624</b> and a balloon <b>626</b> inserted within the stent. In operation, the medical device position guidance system <b>500</b> facilitates locating the position of the coil <b>524</b> and the stent <b>626</b> within the body, as discussed above. After the location of the stent <b>626</b> is assessed, an operator or medical personnel inflates the balloon <b>628</b> by pumping a gas through an air passageway <b>630</b> such as a tube. The inflation of the <b>628</b> deploys the stent <b>626</b> within a vein or artery of the patient. The balloon <b>628</b> is then deflated and balloon <b>628</b>, coil <b>524</b>, wire <b>554</b>, and air passageway <b>630</b> are withdrawn from the vein or artery of the patient.

0066Although the above embodiments relate to positioning an end of a catheter, an ablation therapy device and a stent, it should be appreciated that the medical device position guidance system <b>500</b> is operable to assist the placement of any medical device or invasive component into an animal in the course of stent placement, ablation, blockage removal, heat treatment, surgical procedure, fluid delivery or any other suitable invasive procedure. It should be appreciated that any type of catheter may be used for any of the medical procedures described above. It should also be appreciated that any suitable invasive medical device can be used in place of a catheter.

V. Electronic Configuration for Magnetic Sensing

0067Referring to <figref idref="DRAWINGS">FIGS. 15 to 25</figref>, in an embodiment, the noninvasive device <b>502</b> and catheter assemblies <b>522</b> and <b>523</b> include the components, elements, structure and functionality described above in addition to the components, elements, structure and functionality of the system <b>10</b> described below. System <b>10</b> is used for externally locating a sensor in tissue. The sensor is typically an inductive coil placed within a catheter near its tip. The system <b>10</b> also includes an external, noninvasive device which generates electromagnetic fields which penetrate the patient's skin and couple to the sensor coil. The induced sensor coil voltages are detected. The sensor coil voltages, and the drive signals used to create the electromagnetic fields in the noninvasive device, are compared, to assess the distance between the noninvasive device and the sensor coil, the relative angular orientation, in a horizontal plane, between the catheter and the noninvasive device, and to assess when the noninvasive device is directly over, or very close to, a plane bisecting the center of the sensor coil. The user thus is able to assess the location of the catheter tip, the depth of the catheter in the body, and the direction in which the catheter tip is pointing. This allows the user to confirm that the catheter tip is in the correct location, and pointing in the correct direction, to assist in proper catheter placement.

0068There is shown in <figref idref="DRAWINGS">FIG. 15</figref>, system <b>10</b> according to this embodiment for externally locating a sensor placed in a patient's body. System <b>10</b> includes an noninvasive device which includes pair <b>12</b> of perpendicular electromagnetic output coils. Coil pair <b>12</b> is moved over skin <b>7</b> to detect the depth of, and angular orientation of, inductive sensor coil <b>30</b> carried by and proximate the distal end of catheter <b>9</b> located under skin <b>7</b>.

0069The coils of coil pair <b>12</b> are driven by high frequency signals developed by coil drive voltage generator <b>2</b> under control of microprocessor <b>50</b>. The coil drive voltages are preferably time-multiplexed to allow a single frequency source in microprocessor <b>50</b> to be used to generate the drive signals for both coils.

0070The electromagnetic fields generated from coil pair <b>12</b> penetrate skin <b>7</b> and induce voltages in sensor coil <b>30</b>. These induced signals are transformed, amplified, rectified and multiplexed by transformer/amplifier/rectifier/multiplexer circuit <b>8</b>. A transformer is used to isolate the patient from the input amplifier circuitry. The analog output signal of circuit <b>8</b> is then digitized by analog-to-digital (A/D) converter <b>48</b>. The digitized signals are provided to microprocessor <b>50</b>, which assesses from these signals, and the drive signals provided to coil drive voltage generator <b>2</b>, both the distance between coil set <b>12</b> and sensor coil <b>30</b>, and the direction D (called the “true direction”) in which the distal end of catheter <b>9</b> is pointing. The depth is displayed to the operator by depth display <b>6</b>. The catheter true direction is displayed to the operator by direction display <b>4</b>.

0071A form of coil pair <b>12</b> is shown in more detail in <figref idref="DRAWINGS">FIG. 16A</figref>. Coil pair <b>12</b> includes cross-shaped coil form <b>13</b> on which are wound perpendicular, coplanar coils <b>103</b> and <b>105</b>. Form <b>13</b> may be magnetic material or not. In the drawing of <figref idref="DRAWINGS">FIG. 16A</figref>, the longitudinal axis of coil <b>103</b> lies along an X-axis, and the longitudinal axis of coil <b>105</b> lies along a Y-axis. For convenience of reference hereinafter, coil <b>103</b> will on occasion be referred to as “the X coil,” while coil <b>105</b> will on occasion be referred to as “the Y coil.” Coil set <b>12</b> is shown as being almost directly above sensor coil <b>30</b> of catheter <b>9</b>. Longitudinal axis X of coil <b>103</b> is non-parallel to longitudinal axis Y of coil <b>105</b>. Preferably, the axes are perpendicular. Longitudinal axis B of sensor coil <b>30</b> lies at an angle A from axis X. The direction of the arrowhead on axis B also indicates the direction in which the distal end of catheter <b>9</b> is pointing (the true direction).

0072Coils <b>103</b> and <b>105</b> are driven separately by an X drive voltage and Y drive voltage, respectively, generated by coil drive voltage generator <b>2</b>, <figref idref="DRAWINGS">FIG. 15</figref>. Together, coil drive voltage generator <b>2</b> and microprocessor <b>50</b> shown in <figref idref="DRAWINGS">FIG. 15</figref> alternately energize coils <b>103</b> and <b>105</b>. The distance determinator shown in <figref idref="DRAWINGS">FIG. 15</figref> includes transformer/amplifier/rectifier/multiplexer <b>8</b>, A/D converter <b>48</b>, and microprocessor <b>50</b> which together assess from the voltage induced in sensor coil <b>30</b> the distance between sensor coil <b>30</b> and coils <b>103</b> and <b>105</b>. Preferably, all of the components of system <b>10</b>, with the exception of catheter <b>9</b>, are carried by an noninvasive device (not shown).

0073Coils <b>103</b> and <b>105</b> are alternately energized to generate a time-varying magnetic field which penetrates a patient's skin. In one embodiment, the time-varying magnetic field is created by first driving X coil <b>103</b>, and then driving Y coil <b>105</b> with the same high frequency voltage. Coils <b>103</b> and <b>105</b> are then again sequentially driven by the same voltage, but reversed in phase in relation to the voltage used to drive the coils the first time. This scheme creates a magnetic field whose axis points in sequence to 0°, 90°, 180° and then 270°. This pattern is repeated over and over to create a virtual rotating magnetic field. In another embodiment, which is the embodiment which is employed in the remainder of the description of the embodiments, coils <b>103</b> and <b>105</b> are driven alternately by the same drive voltage, without the phase reversal discussed above. This creates a magnetic field whose axis points in sequence to 0°, 90°, 0°, 90°, etc.

0074Because coils <b>103</b> and <b>105</b> are driven alternately without phase reversal, the voltage induced in coil <b>30</b> is related to both the sensor-to-coil distance, as well as the horizontal angle A of sensor axis B relative to X-coil <b>103</b> axis X, and Y-coil <b>105</b> axis Y. If Vsx is defined as the voltage induced in sensor coil <b>30</b> by the field from coil <b>103</b>, and Vsy the coil <b>30</b> induced voltage from coil <b>105</b>, those values may be determined by the following equations:

0075where

0076Vsx=induced sensor voltage due to field from X coil

0077Vsy=induced sensor voltage due to field from Y coil

0078k=a constant

0079A=horizontal angle between the projection of the axis of the sensor coil and the projection of the axis of the X coil into a plane parallel to the X and Y coils' axes

0080d=distance between sensor and output coils

0081Vsx is thus a maximum when A=0°, and a minimum when A=90°. Conversely, Vsy is maximum when A=90°, and minimum when A=0°. The vector sum of Vsx and Vsy, is independent of angle A. If this vector sum is labeled Vsh, the following holds true:

0082Since Vsh²is in itself a quantity independent of angle A, it is not necessary to calculate the square root of the sum of the squares, as is done in equation (3). By not performing the square root calculation, the number of calculations required by microprocessor <b>50</b>, <figref idref="DRAWINGS">FIG. 15</figref>, is reduced, allowing more time for other calculations to be performed by microprocessor <b>50</b>.

0083Microprocessor <b>50</b> reads and stores the amplified, rectified Vsx and Vsy voltages, and performs the calculations of equations 1 through 3 to develop Vsh or Vsh². As explained below, microprocessor <b>50</b> then puts out digital information to drive depth display <b>6</b>.

0084<figref idref="DRAWINGS">FIG. 16B</figref> is a graph of instrument position versus Vsh. Note that the voltage values on the vertical axis are logarithmic. Shown are measurements taken at sensor coil depths below coil pair <b>12</b> of 2.5 cm, 5 cm, 10 cm, and 15 cm. These appear from the top to the bottom of <figref idref="DRAWINGS">FIG. 16B</figref> in the order just listed. Coil pair <b>12</b> was moved at right angles to the sensor coil longitudinal axis, starting at a position directly over the sensor coil (0 cm), out to 20 cm in either direction from the sensor coil longitudinal axis. As can be seen, the drawing of <figref idref="DRAWINGS">FIG. 16B</figref> illustrates that the induced sensor voltage is maximum when the output coils are directly over the sensor coil. As the coil pair is moved horizontally in a straight line at right angles to the sensor coil axis, the sensor voltage decreases as shown.

0085It is also desirable for the system of this embodiment to assess the true direction D in which the catheter tip is pointing. This is the direction of arrow B, <figref idref="DRAWINGS">FIG. 16A</figref>, which may be defined in relationship to the direction of axis X or axis Y. As shown in <figref idref="DRAWINGS">FIG. 17</figref>, the direction of sensor coil longitudinal axis B (the true direction) may be defined by angle A between axis B and axis X of coil <b>103</b>. Microprocessor <b>50</b> calculates angle A according to the following equation:

0086where A is defined as the horizontal angle between sensor axis B and the X-coil axis X.

0087Angle A may lie in any one of the four quadrants defined by the X and Y axes. In order to assess the true direction in which the catheter tip is pointing, it is necessary not only to calculate the tangent of the angle A, but also to assess into which quadrant sensor coil <b>30</b> is pointing. This determination is made by measuring the phase between the voltage used to drive X coil <b>103</b>, and the Vsx and Vsy voltages. When coil <b>30</b> is pointing to the positive X side of the Y axis, the phase difference between the X coil drive voltage and Vsx is 0 degrees. When output coil <b>30</b> is pointing to the negative X side of the Y axis, there is a 180° phase difference between those two voltages. Similarly, when sensor coil <b>30</b> is pointing to the positive Y side of the X axis, the Y coil drive voltage used to drive coil <b>105</b>, and the voltage induced in the sensor coil from the Y coil voltage, are in phase (0 degrees). When sensor coil <b>30</b> is pointing to the minus Y side of the X axis, those two voltages are out of phase (180 degrees). Thus, by making the two phase comparisons, the quadrant is assessed, which then fully defines the direction of longitudinal axis B in relation to longitudinal axis X, thus determining the catheter distal end true direction.

0088<figref idref="DRAWINGS">FIG. 18A</figref> is a cross sectional view through a embodiment of the noninvasive device <b>220</b> and the catheter <b>9</b> including sensor coil <b>30</b>, of the system of this embodiment. <figref idref="DRAWINGS">FIG. 18A</figref> introduces an additional concept of a embodiment of the system of this embodiment. Noninvasive device <b>220</b> includes horizontal coil pair <b>12</b> as described above relative to <figref idref="DRAWINGS">FIGS. 16A and 17</figref>. Also included is vertically-oriented electromagnetic coil <b>122</b> which lies along “vertical” longitudinal axis <b>123</b>, i.e., an axis transverse to a plane parallel to the axes of coils <b>103</b> and <b>105</b>. <figref idref="DRAWINGS">FIG. 18B</figref> shows an alternate way of constructing coil set <b>12</b><i>a </i>to incorporate vertically oriented coil <b>122</b><i>a </i>and horizontal coils <b>103</b><i>a </i>and <b>105</b><i>a</i>. In use, noninvasive device <b>220</b> is held so that rounded noninvasive device tip <b>223</b> is on or next to the skin surface <b>221</b>. As explained below, the noninvasive device is moved across surface <b>221</b> to locate sensor coil <b>30</b> near the distal end of catheter <b>9</b>. As explained in detail above, the transmitter <b>516</b> wirelessly emits radio waves through antenna <b>560</b> which are then received by the receiver <b>512</b>. The receiving processor <b>523</b>, as illustrated in <figref idref="DRAWINGS">FIG. 18A</figref>, receives information, transmitted by the transmitter <b>516</b> of the transmitter assembly <b>558</b>, and the receiver processor <b>523</b> converts this information from a sinusoidal electromagnetic wave having a determined modulated frequency to a series of electrical impulses. The receiver processor <b>523</b> send these impulses to the main processor <b>517</b> through one or more pins within the receiver <b>512</b>. The main processor <b>517</b> processes these impulses to cause the indicator <b>566</b> of the noninvasive device as illustrated in <figref idref="DRAWINGS">FIG. 5</figref> to provide visual output. Operator displays <b>222</b> mounted in noninvasive device <b>220</b> are described in more detail below.

0089Vertical coil <b>122</b>, <figref idref="DRAWINGS">FIG. 18A</figref>, is used to assess when noninvasive device tip <b>223</b> is at or very close to plane P shown in <figref idref="DRAWINGS">FIG. 19A</figref>, which bisects sensor coil <b>30</b> and is perpendicular to longitudinal axis <b>124</b> of sensor coil <b>30</b>. This embodiment of the system includes a sensor coil position determinator for determining, from the vertical output coil drive voltage, and the electrical voltage induced in sensor coil <b>30</b> by this vertical coil drive voltage, when the longitudinal axis <b>123</b> of vertical coil <b>122</b> is proximate plane P. This assesses more exactly the position of sensor coil <b>30</b> in relation to noninvasive device <b>220</b>. This determination can be made in two ways. The first way is to measure the phase change of the induced sensor voltage.

0090As vertical coil <b>122</b> moves from one side of plane P to the other, the phase of the voltage induced in sensor coil <b>30</b>, in relation to the phase of the high frequency drive signal used to drive coil <b>122</b>, changes from 0° (in phase) to 180° (out of phase). For example, when coil <b>122</b> is at position <b>122</b><i>a </i>on one side of plane P, <figref idref="DRAWINGS">FIG. 19A</figref>, the two signals are in phase. When coil <b>122</b> is at position <b>122</b><i>c </i>on the other side of plane P, the signals are out of phase. When coil <b>122</b> is at position <b>122</b><i>b</i>, in which longitudinal axis <b>123</b> of coil <b>122</b> lies in plane P, there would be no signal. However, in reality the induced sensor signal in this case would very quickly alternate between being in phase and out of phase with the output coil drive signal, as due to movement of the operator's hand, and slight movements of the sensor coil in the patient, coil <b>122</b> would never actually remain exactly centered on plane P.

0091The second way to assess the position of sensor coil <b>30</b> in relation to noninvasive device <b>220</b> is to measure the change in amplitude of the induced sensor voltage. It has been found that the amplitude of the induced sensor voltage from the field generated from the vertical coil drops to a minimum, or a null, when the coil is directly over the plane P, (position <b>122</b><i>b</i>). As shown in <figref idref="DRAWINGS">FIG. 19B</figref>, the induced sensor voltage Vsv drops nearly to 0 when coil <b>122</b> is positioned in plane P. At a sensor coil depth of 10 cm, the voltage increases up to approximately 40 millivolts as the vertical coil is moved horizontally along axis <b>124</b>, <figref idref="DRAWINGS">FIG. 19A</figref>, approximately 5 cm from plane P. Thus, the positioning of the vertical coil in relation to the sensor coil can also be assessed from the sensor coil output voltage. Detection of plane P may thus be based either on the phase change between the vertical coil drive voltage and the resulting induced sensor voltage, or by detection of the sensor voltage null. Null detection, the embodiment, is described in relation to <figref idref="DRAWINGS">FIGS. 20 through 23</figref>.

0092<figref idref="DRAWINGS">FIG. 20</figref> is an electronic schematic diagram of the embodiment of the system of this embodiment. Horizontal output coils <b>103</b> and <b>105</b> are wound on cross shaped core <b>12</b>. Also shown is vertically-oriented coil <b>122</b>. The coils are driven sequentially as shown in <figref idref="DRAWINGS">FIG. 21</figref>. Signal F is a high frequency drive voltage derived by dividing the clock frequency of microprocessor <b>50</b> using frequency divider <b>70</b>. In one embodiment, the microprocessor <b>50</b> clock frequency is 2 Mhz, and the divider ratio is 32, resulting in a frequency of signal F=62.5 kHz. The sequence of coil drives is established by the output coil driver means which includes microprocessor <b>50</b>, frequency divider <b>70</b>, exclusive OR circuit <b>72</b>, and amplifiers <b>74</b>, <b>76</b>, <b>78</b> and <b>80</b>. Microprocessor <b>50</b> also has control outputs labeled X, Y, and V, shown in <figref idref="DRAWINGS">FIG. 21</figref>. These control signals are provided to circuit <b>72</b> to result in multiplexed high frequency drive signals which are amplified and provided to the appropriate coil as the X coil, Y coil and V coil currents depicted in <figref idref="DRAWINGS">FIG. 21</figref>. The vertical coil is thus energized after each time that either the X coil or Y coil is energized. The resulting voltages induced in sensor coil <b>30</b> are also shown in <figref idref="DRAWINGS">FIG. 21</figref>. In this example, the sensor voltage induced by the X coil current is larger than that induced by the Y coil current. For other sensor coil directions, the sensor voltage induced by the X coil current may be smaller than or equal to that induced by the Y coil current.

0093The induced sensor voltage is coupled through isolation transformer <b>32</b> to amplifier <b>34</b>, band pass filter <b>35</b>, full wave rectifier <b>36</b>, low pass filter <b>38</b>, and DC amplifier <b>40</b>. Zero adjustment <b>39</b> ensures that the output of amplifier <b>40</b> is 0 volts when the sensor is positioned remotely from all three of the output coils, at a point where virtually 0 voltage is induced in the sensor. The output of amplifier <b>40</b> is connected to multiplexer <b>46</b>, whose timing is controlled by signals M<b>1</b> and M<b>2</b> from microprocessor <b>50</b>. The multiplexing scheme is described below in conjunction with <figref idref="DRAWINGS">FIG. 23</figref>. The multiplexer output is connected to amplifier <b>47</b> which has a gain of 1. These components make up transformer/amplifier/rectifier/multiplexer circuit <b>8</b><i>a</i>. The rectified, filtered output signal of amplifier <b>40</b> is shown in the lowermost graph of <figref idref="DRAWINGS">FIG. 21</figref>.

0094As shown in <figref idref="DRAWINGS">FIG. 22</figref>, which is a graph of the output of multiplexer <b>46</b>, the multiplexer is preferably timed to connect the amplifier <b>40</b> output voltage to amplifier <b>47</b> and then on to A/D converter <b>48</b> for the greater part of the measurement cycle. Multiplexer <b>46</b> is periodically connected to the battery voltage Vbat and precision DC voltage reference Vref, and to test sensor voltage Vts. In one embodiment, these three voltages are measured in sequence, once per second over three consecutive operating cycles. If the battery voltage Vbat drops below a predetermined threshold, microprocessor <b>50</b> is programmed to turn on a low battery indicator light. If the precision voltage reference Vref source changes value beyond a small tolerance, the microprocessor is preferably programmed to turn the instrument off.

0095Test sensor <b>130</b>, <figref idref="DRAWINGS">FIG. 20</figref>, consists of a small inductive coil positioned adjacent to all three output coils. Typically, but not necessarily, its longitudinal axis is at a 45° angle to the longitudinal axes of all three output coils. The fields from each of the three output coils induce voltages in the test sensor which are amplified, rectified and filtered by amplifier <b>132</b>, rectifier <b>134</b>, and low pass filter <b>136</b>, respectively. The resulting voltage Vts is periodically read by microprocessor <b>50</b>. If an output coil should break, or if the coil drive current should fail or decrease beyond a preset limit, the test sensor output voltage Vts would change accordingly. The microprocessor is programmed to sense this and turn the instrument off such that the instrument is on only when functioning properly.

0096The digital output of A/D converter <b>48</b> is connected to microprocessor <b>50</b>. Microprocessor <b>50</b> is programmed to store the three voltage levels Vsx, Vsy and Vsv and perform the appropriate calculations to assess the sensor depth (distance from output coils <b>103</b> and <b>105</b> to sensor coil <b>30</b>) and the true direction determined from angle A, <figref idref="DRAWINGS">FIGS. 16A and 17</figref>, as described above. The calculated values are then displayed as outputs to the operator. The forms of the outputs are shown in <figref idref="DRAWINGS">FIG. 6</figref> and also <figref idref="DRAWINGS">FIGS. 24A through 25</figref>.

0097Data establishing the sensor depth may be provided on four data lines to both decoder driver <b>52</b> and binary to seven-segment decoder driver <b>62</b>. Driver <b>52</b> is enabled to drive light bar display <b>48</b>, which may indicate the strength of the induced sensor voltage Vsh, shown in FIG. <b>16</b>B. As illustrated in <figref idref="DRAWINGS">FIGS. 24A and 24B</figref>, light bar display <b>48</b><i>a </i>may be mounted on the upper surface of hand held noninvasive device <b>220</b><i>a </i>and include a number of segments which are typically LEDs with a scale of centimeters alongside. The minimum distance, which corresponds to the peak signal strength, is in one embodiment, continuously updated and the corresponding LED lit, along with the LED representing the currently-sensed distance, to give the operator a better idea of when the noninvasive device is closest to or directly over the sensor coil. Alternatively, numerical display <b>66</b>, also shown as display <b>66</b><i>a</i>, <figref idref="DRAWINGS">FIG. 25</figref>, may be used to indicate the depth directly in inches or centimeters. The system converts Vsh (or Vsh²) to distance by using the value of the variable to address a distance lookup table in microprocessor <b>50</b>, <figref idref="DRAWINGS">FIG. 20</figref>. The lookup table stores numbers which convert to the depth (in inches, centimeters, or audio frequency).

0098Microprocessor <b>50</b> may also produce a variable frequency which is related to the induced sensor voltage and which is used to drive amplifier <b>42</b>, which drives speaker <b>44</b> through volume control <b>43</b>. This provides a tone whose frequency changes relative to the induced sensor voltage.

0099Direction display <b>78</b> in this embodiment consists of eight LEDs arranged in a circle as shown in <figref idref="DRAWINGS">FIG. 24A</figref> as direction display <b>78</b><i>a</i>. These LEDs are driven by decoder-driver <b>76</b>, which converts digital information from microprocessor <b>50</b> to energize the appropriate direction-indicating LED such as LED <b>227</b>, <figref idref="DRAWINGS">FIG. 24A</figref>. This direction display indicates that the distal end of the catheter is pointing in the direction of LED <b>227</b>. This is the true direction in which the catheter distal end is pointing. This information is derived from the determination of angle A as described above in conjunction with <figref idref="DRAWINGS">FIGS. 24 and 27</figref>.

0100<figref idref="DRAWINGS">FIG. 23</figref> is a flow diagram of the embodiment of the program resident in microprocessor <b>50</b>, detailing the steps used to drive the three output coils, collect the sensor voltage level, make the necessary calculations and comparisons, and drive the audible and visual displays described above.

0101Start step <b>300</b> initializes all the storage registers and timers to zero, or to appropriate start up values. In step <b>301</b>, multiplexer <b>46</b> is set by microprocessor <b>50</b> to select the reference DC voltage reference V_REF, and then select the amplified, rectified output of test sensor <b>130</b>, Vts, and sequentially connect the voltages to A/D converter <b>48</b> shown in <figref idref="DRAWINGS">FIG. 20</figref>. If the values of the two voltages fall within preset limits, operation continues to step <b>302</b>. Otherwise, a failure is indicated by alternately flashing the ON LED and the BATT.LOW LED for 15 seconds. The instrument is then shut off. In step <b>302</b>, multiplexer <b>46</b> is connected by microprocessor <b>50</b> to the battery voltage Vbat. If Vbat is below a preset level, a low battery indicator, or LED, is activated by microprocessor <b>50</b>. Operation would then continue to step <b>303</b>.

0102In step <b>303</b>, microprocessor <b>50</b> selects line X shown in <figref idref="DRAWINGS">FIG. 20</figref> to drive X coil <b>105</b>. Multiplexer <b>46</b> is enabled to select the amplified and rectified induced sensor voltage at the output of amplifier <b>40</b>, which is digitized and stored in the memory of microprocessor <b>50</b>. Steps <b>304</b>, <b>305</b> and <b>306</b> repeat the process for the vertical coil by selecting line V out of the microprocessor <b>50</b>, the Y coil by selecting line Y out of the microprocessor <b>50</b>, and the vertical coil a second time, respectively.

0103In step <b>307</b>, the signals induced by the X and Y coils are squared and summed by microprocessor <b>50</b> to produce a value which is based on the strength of the X and Y fields at the sensor, regardless of the sensor-to-output coil horizontal angle. In step <b>308</b>, microprocessor <b>50</b> outputs from its lookup table digital information to drive decoder drivers <b>52</b> and <b>62</b> using the calculation as described above. At step <b>309</b>, microprocessor <b>50</b> converts the values derived in step <b>307</b> to a variable frequency tone which drives speaker <b>44</b>.

0104Steps <b>310</b> through <b>317</b> are the sensor coil location determination and direction display steps. In step <b>310</b>, microprocessor <b>50</b> reads the value of the sensor coil voltage induced by the field generated from vertical output coil <b>122</b>. Step <b>311</b> constitutes the microprocessor null detection subroutine for detecting the null in the output as shown in <figref idref="DRAWINGS">FIG. 19B</figref>. The microprocessor periodically reads and stores the peak value of the sensor voltage induced by the field from vertical output coil <b>122</b>. Microprocessor <b>50</b> has established therein a threshold value which is a specific percentage of the peak vertical induced voltage. Typically, this threshold is set as ¼ of the peak voltage. Since the peak is continuously refreshed, this threshold may change. When the sensed voltage drops below this threshold, the microprocessor enters a second state—called state <b>2</b>. After entering into the second state, if the vertical induced voltage again rises above the threshold by a predetermined amount, for example 50% above the threshold, state <b>3</b> is entered in step <b>311</b>. If state <b>3</b> is entered, at step <b>312</b> the value of Vsh²is read. If the sensor-to-output coil distance determined from Vsh²is greater than a preset value (in this case 18 cm) the direction display is inhibited. This prevents the direction calculation from being based on weak induced signals which may have a large noise component and thus be inaccurate. In step <b>313</b>, the microprocessor determines whether Vsh²exceeds the peak value of Vsh²divided by 4. If so, the tangent of angle A is calculated, step <b>314</b>, the quadrant into which the sensor coil is pointing is determined, step <b>315</b>, the appropriate direction-indicating LED is lit, step <b>316</b>, and a “sensor found” audible beep is generated, step <b>317</b>. This indicates to the operator that the sensor has been found. The direction calculation is performed only when the Vsv null has been detected (state <b>3</b>). This is such that the Tangent A is calculated only when the XY output coil pair is closest to the sensor and at or near the plane of the sensor midpoint. This is where the tangent calculation is the most accurate.

0105As can be seen from <figref idref="DRAWINGS">FIG. 19B</figref>, if noninvasive device <b>220</b>, which contains vertical coil <b>122</b>, is moved back and forth relatively quickly while held at about 15 to 20 cm horizontally removed from the bisecting plane, at which there is a relative null in voltage V_sv, a false null may be simulated. That is, the voltage V_svcan drop below the threshold and then rise again a percentage above the threshold. To reduce the likelihood of such a false null determination, microprocessor <b>50</b> is preferably programmed to require state <b>3</b> to occur within a specific required time interval after state <b>2</b> is entered, or else null detection is inhibited.

0106This state <b>2</b> to state <b>3</b> time interval is preferably variable with the strength of the peak voltage. For large sensor to output coil distances (depths), the peak signal is weak and the null is wide. That is, the voltage drops off relatively gradually as the vertical coil approaches plane P. In that case, a relatively long time interval is needed to allow the operator to move the instrument a sufficient distance to reach state <b>3</b>. On the other hand, at shallow depths, the null becomes sharp and narrow. That is, the voltage drops off very rapidly when the output coil is very close to plane P. In this case, since the distance the noninvasive device must traverse to reach state <b>3</b> is small, the time interval can be short.

0107<figref idref="DRAWINGS">FIG. 19B</figref> illustrates this concept for three different catheter depths. At a depth of 10 cm, the noninvasive device must move from point A to point B to enter state <b>3</b>. This equates to a distance of approximately 3 cm. If the noninvasive device is typically moved at 10 cm per second, the time interval to reach state <b>3</b> should be at least 0.3 seconds. At a depth of 20 cm, the distance from point C to point D is about 7 cm, which requires 0.7 seconds. Thus, the time interval should be at least 0.7 seconds. At shallow depths of 5 cm, only about 0.15 seconds is needed to traverse from point E to point F at the indicated speed. Thus, the time interval after state <b>2</b> is entered in which state <b>3</b> must be entered is preferably variable from about 0.15 to about 1.0 seconds. This time interval may be established by software in the microprocessor according to the peak value of Vsv using a lookup table.

0108In addition, the stored peak values of Vsh²and Vsv are preferably made to decay at a specific time constant, typically between 0.3 and 2.0 seconds. Decaying the Vsv peak helps to reduce false null determinations by continuously reducing the threshold values at distances remote from the sensor. If the decay time constant is too short, null detection can be inhibited if the operator is moving the instrument too slowly. If the decay constant is too long, false nulls can be indicated, if the operator moves the instrument back and forth at a horizontal distance of perhaps 15 to 20 cm from the sensor coil. Preferably the Vsh²peak is also decayed in a similar manner such that null detection will not be inhibited if the operator should move the instrument slightly farther vertically from the sensor, thereby reducing Vsv while the same threshold voltage is maintained.

0109In the embodiments described above, the noninvasive device <b>502</b>, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, for example, is able to communicate with the catheter assembly <b>504</b> through a changing magnetic field and also receive information relating the induced magnetic field in the coil <b>524</b>. Therefore, the noninvasive device <b>502</b>, which, in one embodiment, has no connecting wires or power cords, may be at least temporarily contained within a sterile bag or sheet (not shown) to protect the patient from substantial contamination that may exist with respect to the noninvasive device. Accordingly, the noninvasive device may be reused over a series of procedures with different patients while still facilitating a substantially sterile environment. Also, the lack of wires attached to the external housing of the noninvasive device <b>502</b> reduces the likelihood of wires becoming tangled with the medical personnel or other equipment during a medical procedure.

0110Although specific features of this embodiment are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the embodiment. It should be understood that various changes and modifications to the presently embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.